Measuring devices designed for continuously tracking a target point and a coordinative determination of the position of this point can be summarized under the term laser tracker, particularly in the context of industrial measurement. In this, a target point may be represented by a retro-reflecting unit (e.g. cubic prism) that is targeted with an optical measurement beam of the measuring equipment, particularly a laser beam. The laser beam is reflected parallel to the measuring equipment, with the reflected beam being acquired with the acquisition unit of the device. In this, an emission and a receiving direction of the beam respectively, for instance with the help of sensors for angle measurement assigned to a beam splitting mirror or a targeting unit of the system, is determined. Moreover, a distance from the measuring equipment to the target point is determined when acquiring the beam, e.g. by means of the time-of-flight or phase difference measurement or by using the Fizeau principle.
State-of-the-art laser trackers may additionally be designed with an optical picture acquisition unit with a two-dimensional, light-sensitive array, e.g. a CCD or CID camera or a CMOS array-based camera, or a pixel array sensor and with a picture processing unit. In this, the laser tracker and the camera may particularly be mounted one on top of the other in such a way that their positions relative to each other cannot be changed. For instance, the camera can be pivoted together with the laser tracker around the laser tracker's essentially vertical axis, but can be pivoted upwards and downwards regardless of the laser tracker and hence is positioned separately from the lens of the laser beam in particular. Furthermore, the camera—e.g. depending on the respective application—may be designed in such a way that it can only be pivoted around a single axis. In alternative implementations, the camera may be mounted together with the laser lens in a common housing in an integrated design.
The processes of acquiring and interpreting a picture—by means of picture acquisition and picture processing unit—of so-called auxiliary measuring equipment with marks, the relative length of which regarding each other is known, are indicative of a spatial orientation of an object (e.g. a probe) positioned at the auxiliary measuring equipment. Together with the determined spatial position of the target point, it is furthermore possible to precisely determine the spatial position and the orientation of the object absolutely and/or relative to the laser tracker.
Such auxiliary measuring equipment may be embodied by so-called touch tools positioned on one point of the target object with their contact point. The touch tool is characterized by marks, e.g. points of light, and a reflector representing a target point at the touch tool and that can be targeted with the laser beam of the tracker, with the positions of the marks and the reflectors relative to the contact point of the touch tool being known precisely. In a manner known to the person skilled in the art, the auxiliary measuring equipment may also be, for instance, a hand-held scanner equipped for distance measurement for non-contact surface measurements, with the direction and position of the scanner measurement beam used for distance measurement relative to the light points and reflectors positioned on the scanner being known precisely. Such a scanner is described in EP 0 553 266, for instance.
For distance measurement purposes, state-of-the-art laser trackers are equipped with at least one distance measurement unit, with this possibly being present as an interferometer, for instance. Since such distance measurement units are only capable of measuring relative changes regarding the distance, so-called absolute distance measurement units are installed in today's laser trackers, in addition to interferometers. The interferometers used for distance measurement in this context mainly use HeNe gas lasers as sources of light—due to the large coherence length and the measurement range facilitated by this length. In this, the coherence length of the HeNe laser may be several hundred meters so that the ranges required in the field of industrial measurement technology can be achieved using relatively simple interferometer arrangements. For instance, a combination of an absolute distance measurement unit and an interferometer for determining the distance using an HeNe laser is known from WO 2007/079600 A1.
Furthermore, a fine targeting sensor is used in advanced tracker systems—increasingly standardized—to determine a deviation of the received measurement beam from a zero position. Using this measurable deviation, it is possible to determine a position difference between the center of a retro-reflector and the point of impact of the laser beam on the reflector and to correct and reposition, respectively, the orientation of the laser beam depending on this deviation in such a way that the deviation on the fine targeting sensor is reduced, particularly is “zeroed”, so that the beam is oriented towards the center of the reflector. By repositioning the orientation of the laser beam, continuous tracking of the target point can be implemented and the distance and position of the target point can be determined continuously relative to the measuring equipment. In this, repositioning may be implemented by changing the orientation of the beam-splitting mirrors moved in a motorized manner and designed for deflecting the laser beam and by pivoting the targeting unit that is equipped with the beam-guiding laser lens, respectively.
The target tracking process described must be preceded by the process of coupling the laser beam to the reflector. For this, an acquisition unit for target finding with a position-sensitive sensor and with a relatively large field of view may additionally be positioned at the tracker, with the optical sensor axis defined by the sensor and the axis, along which the measurement laser beam extends, are offset to one another. Furthermore, generic equipment includes additional means of illumination that are used to illuminate the target and the reflector, respectively, particularly with a defined wavelength differing from the wavelength of the distance measuring equipment. In this context, the sensor may be designed in such a way that it is sensitive to a range around this determined wavelength, for instance in order to reduce or completely prevent extraneous light effects. By means of the means of illumination, the target can be illuminated and the camera can be used to acquire a picture of the target with illuminated reflector. By showing the specific (wavelength-specific) reflex on the sensor, the reflex position in the picture can be resolved and hence an angle relative to the acquisition direction of the camera and a direction to the target and reflector, respectively, can be determined. One embodiment of a laser tracker with such a target tracking unit is known from WO 2010/148525 A1, for instance.
Depending on the direction information that can be derived in the manner above, the orientation of the measurement laser beam can be changed in such a way that a distance between the laser beam and the reflector, the laser beam is to be coupled to, is reduced. Due to the offset regarding the optical sensor axis defined by the sensor and the measurement axis of the measuring equipment, the beam can be directed towards the target with the help of the sensor-based determination of the direction to the target, and therefore coupling cannot be performed within one direct step. For a stationary target, this requires several iteration steps with one measurement process in each case (re-determination of a direction to the target using the sensor) in order to approximate the laser beam. As a consequence, the disadvantage of such an approximation method is that tracking and targeting the target are time-consuming processes (since they are iterative) and that tracking, particularly in the event of a movement of the target relative to the sensor, is not robust and unambiguous. Furthermore, no approximation of the laser beam regarding the target can be achieved in the event of a movement of the target relative to the laser tracker, since a deviation between the target detected using the sensor and the laser beam changes continuously in so doing. As a consequence, no iterative approximation of the beam regarding the target may be achieved due to this change regarding the deviation occurring during the movement of the target. In this, every iteration step comprising the re-acquisition of a reflex corresponds to such a first measurement regarding a (new) target. In general, this results in a huge disadvantage of such systems for target tracking consisting in the fact that stationary targets can only be targeted in a relatively time-consuming manner and that it is not possible at all to directly target moving targets.